Switch recently added man-db to keg only; not GNU, muscle-memory

[RalphCorderoy]

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A formula for man-db was recently added by @maxim-belkin. It configures a prefix g so the resulting gman, etc., don't clash with the system's. I think it would be better for the formula to drop the prefix and to be keg only so the user then has a choice how to access the command. man-db is not a GNU project, the normal meaning of the g prefix, and man(1) is such a common command that muscle-memory types man ls and doesn't want re-training when non-Homebrew systems are still being used. Because man groff' will use the old, crufty, system man the user might take a while to notice issues, realise the problem, and have to re-start with gman groff'. I think most would prefer to make the explicit choice after brewing man-db to make its man their default.

AIUI, once keg only, after brew install man-db the user is prompted with the edits to make to their profile to add the new executables' directory to their PATH. They could also choose another avenue, like symlinking into ~/bin, or a bash alias, etc.

I'm not a Mac/Homebrew user so sorry if I have some of the terminology wrong. I've an interest in man-db, and the underlying man-page formatter. This issue resulted from a private email discussion involving Maxim, among others.

[jonchang]

brew info man-db says:

==> Caveats
Commands also provided by macOS have been installed with the prefix "g".
If you need to use these commands with their normal names, you
can add a "bin" directory to your PATH from your bashrc like:
  PATH="/usr/local/opt/man-db/libexec/bin:$PATH"

So you can add this to your PATH variable if you'd like to use man-db by default.

[RalphCorderoy]

Thanks @jonchang. Is something similar shown on installation?
It doesn't cover the g prefix being used for a non-GNU package. Is the g prefix commonly used by other formulas, and are any of them non-GNU too?

The discussion on the original #36469 started with keg-only being the way to do it, and only deviated after objections from the PR's author. g was one solution grasped. I think the original idea was a better fit.

[maxim-belkin]

The above "caveats" message does show up upon installation, so careful people should know what to do. I strongly agree that g prefix in this situation is at odd and inconsistent but is what is used in other formulae. I don't know if any other non-gnu formula uses g prefix. I suggested a few alternatives but ... you know, no one other than myself expressed this concern and we had to reach a consensus within a reasonable amount of time.